Heretofore, an example of the air cleaner of this type is constructed as shown in FIG. 10. That is, in a case 81, an ionization unit 84, which comprises ionization wires 82 and ionization electrodes 83, and a dust-collecting electrode unit 87, which comprises dust-collecting electrodes 85 and auxiliary electrodes 86, are provided. In the ionization unit 84, D.C. high voltage is applied from a D.C. high voltage source 100 across each of the ionization wires 82 and each of the ionization electrodes 83, and thereby a corona discharge is made, and dusts are ionized. Ionized dusts are transferred to rear part by means of a fan 88 and pass through the dust-collecting electrode unit 87. In the dust-collecting electrode unit 87, D.C. high voltage is applied from the D.C. voltage source 100 across the dust-collecting electrodes 85 and the auxiliary electrodes 86, and thereby charged dusts are attached on the dust-collecting electrodes 85. However, since each gap between both electrodes 85 and 86 is large, size of the dust-collecting electrode unit 87 undesirably becomes large.
In recent years, a dust-collecting electrode unit as shown in FIG. 11 is proposed to overcome the shortcoming of the above-mentioned dust-collecting electrode unit 87.
That is, films, each of which comprises a first conductive layer 92 provided on a surface of a first insulation layer 91, and films, each of which comprises a second conductive layer 94 provided on a surface of a second insulation layer 93, are alternately laminated with every uniform spacial gaps formed therebetween. Arrows show flowing direction of air.
In the above-mentioned construction, the principle for collecting dusts is described hereafter. In a state such that positive high potential is applied to the first conductive layer 92 and the second conductive layer 94 is grounded, when the dusts, which are charged with positive electricity at a front side of the dust-collecting electrode unit, pass through the dust-collecting electrode unit, the dusts are attached on a surface of the conductive layer 94 of grounded potential and a surface of the second insulation layer 93 by force of Coulomb's law in the electric field, thereby performing dust-collection. The dusts, which are charged with positive electricity and attached on the conductive layer 94 of grounded potential, are electrically neutralized, however, the positive-charged dusts which are attached on the second insulation layer 93 cannot be neutralized, thereby resulting in a state such that the dusts are charged with positive electricity on a surface of the second insulation layer 93. These positive-charged electric charges which are attached on the surface of the second insulation layer 93 act to weaken electric field within each of the spacial gaps between the first conductive layer 92 impressed with positive high potential and the second insulation layer 93, thereby resulting in an undesirable state such that the force of Coulomb's law is weakened and a dust-collecting ratio rapidly lowers as time passes. The above-mentioned description is made with regard to the dusts which are charged with positive electricity at the front side of the dust-collecting electrode unit, but, even when dusts which are charged with negative electricity at the front side of the dust-collecting electrode unit pass through the dust-collecting electrode unit, similar problems will occur.